Variable speed belt drives normally have two variable pitch driven sheaves wherein one of the flanges of each sheave is mounted on and fixed to a shaft and the other of the flanges is slidable on the shaft toward and away from the fixed flange to vary the pitch and hence change the output speed of the drive. In conventional sheaves of this type, the two flanges of the drive sheave are keyed to the shaft and the torque from the motor is transmitted through the shaft and key to the two flanges, and in most drives the shaft is connected directly to and supported by the shaft of the drive motor; however, in some variable speed drives the shaft of the drive sheave is journaled in pillow blocks at the ends and is connected to the motor shaft by a coupling. The sheave and motor shafts may be connected by a flexible coupling having flanges mounted on the two shafts in spaced end-to-end relation and connected by a flexible torsion element which compensates for minor misalignment between the two shafts. This type of shaft connection requires a substantial amount of space in order to assemble the two shafts and the coupling beyond and in spaced relation to the pillow block, thus making the overall size of the drive relatively large and occasionally too large for a particular installation or application, such as in air conditioner housings and similar equipment.
An alternative to the drive coupling and shaft assembly described above is a direct system in which the sheave shaft is connected directly to and supported by the motor shaft; however, this system has the disadvantage of being extremely difficult to obtain satisfactory shaft alignment, including alignment between the motor mounted constant speed shaft of the drive sheave and the separately mounted shaft of the variable speed sheave. These two shafts must be made parallel and square between the belt and sheave in three planes within very close tolerances, after the parts are installed in the machine to be driven. Since precise and reliable test equipment for making satisfactory shaft alignment, and the expertise and the time required to make proper alignment and adjustments, are rarely available at the installation site, these direct motor driven drives basically have a short belt and sheave life resulting from misalignment.